EXPLORING EVIDENCE THAT MANY, PERHAPS MOST, BASINS OF THE SUBMERGED FOREARC ARE CREATED BY SUBCRUSTAL SUBDUCTION EROSION

INTRODUCTION:It is widely recognized that large forearc basins are associated with subduction zones (SZs) where a large accretionary wedge frontally extends the upper plate to the trench. Construction of the wedge forms the relief of the forearc basin by seaward uplift or critical-wedge adjustment of the tectonically compiled body. Example basins are the Cretaceous Great Valley of California and the still forming Tobago Trough of the Lesser Antilles and the Java Basin of western Java. But most forearc basins occur along convergent margins where frontal accretion is not a governing process and coastally exposed basement thins seaward and extends to within a few tens of km of the trench. So how are these basins formed?

OBSERVATIONS: For the Pacific rim, marine geophysical imagery and drilling, dredging, and submersible observations document that the prominent forearc basins of the Tonga-Kermadec, IBM, Japan, Kuril, Kamchatka, Alaska-Aleutian, Cascadia, Central America, and western South America from Colombia to Tierra del Fuego are underlain by coastally exposed basement rock of early Tertiary to Proterozoic age. Characteristically, the subsurface basement profile of these basins is smooth and spoon-shaped rather than one bordered by prominent boundary faults. And, characteristically, the basins are underlain by thinned forearc basement.

INFERENCES: Based on the above observations and the circumstance that these Pacific-rim sectors are, over the long term, dominantly erosive margins (i.e., the margin progressively subsides seaward and narrows toward a fix onshore point), it is inferred that their forearc basins are a manifestation of differential basal subduction erosion. Areally large and structurally deep basins may express the subcrustal erosive effects of shallow-angle subduction or the underunning of wide and high basement relief, e.g., where the Nazca and Juan Fernandez Ridge obliquely enter the Chile SZ, or the Louisville Ridge the Tonga SZ. Small isolated basins may reflect the subsurface passage of a high-relief edifice that initiated a focal area of enhance subcrustal erosion. Although proposed, it seems unlikely that the structural origin of inland forearc basins formed on crust overlying the mantle wedge, e.g., the Puget-Willamette lowlands, are a consequence of subduction erosion.